There are a variety of methods known in the prior art for producing particulate, urea-containing compositions. In the past, urea particles have been produced customarily by means of spray crystallization, where a substantially water-free urea melt (water content from 0.1 to 0.3 wt %) is sprayed from the upper part of a spray crystallization tower into an ascending stream of air at ambient temperature, with the droplets solidifying to form crystals (prills). The diameters of the resultant prills are relatively small, and their mechanical strength is low.
Further known methods for producing particulate, urea-containing compositions use drum or plate granulating units, or else what are called spherodizer drums, for producing the particles.
Urea particles having larger particle diameters and better mechanical properties are nowadays usually produced by granulation of a substantially water-free urea melt or of an aqueous urea solution in a fluidized bed, as described for example in U.S. Pat. No. 4,219,589. In these granulation methods, an aqueous urea solution having a urea concentration of 70-99.9 wt % is introduced, in the form of very finely dispersed droplets having an average diameter of 20-120 μm, into a fluidized bed of urea particles, the temperature being selected such that the water in the solution sprayed onto the urea particles undergoes evaporation and urea is deposited on the particles, thus producing granules having a desired particle size of 2.5 mm or more.
Because this method produces relatively large quantities of fine dust, particularly if the urea solution used has a water content of more than 5 wt %, it is common to employ granulating additives which reduce this dusting. The result of adding these additives is that the granular particles and particularly their surface remain plastic, meaning that round particles having a smooth surface and good mechanical stability are obtained as a result of their rolling movements and collisions.
The resulting granules therefore have high compressive strength and impact resistance, a low tendency toward dusting through abrasion, and also, even on prolonged storage, only a low tendency toward caking. Such granulating additives are employed, however, not only in fluidized bed granulation, but also in other methods referred to above.
Granulating additives employed are customarily formaldehyde or water-soluble adducts and/or condensation products of formaldehyde and urea, but must be added in relatively large quantities and have toxicity properties which render their handling not unproblematic. Formaldehyde emissions pose an acute risk to health and environment, although the introduction of urea-formaldehyde prepolymers such as UF80 or UF85 has reduced such risks. Moreover, the issue of health risks also arises in connection with chronic exposure to formaldehyde vapors, which cannot be avoided entirely even by the use of such prepolymers.
A further problem affecting the granulation of urea-containing particles is the production of dust, referring here to particles having a diameter of less than 0.5 mm. This production of dust is attributable substantially to three sources. A first is the abrasion of the granules owing to movements and collisions of the particles, in the fluidized bed, for example, with the amount of dust produced being substantially dependent on the mechanical properties of the granules. Furthermore, the nozzles or the liquid distributors used in the other methods each generate drops with a certain distribution of diameters, with the finest drops then solidifying before they strike the urea particles, meaning that the dust thus formed leaves the granulator again with the outgoing air. Lastly, a third source is the dust resulting from the comminution of oversized granular particles, this dust customarily being transferred directly into the granulator again in the methods and units according to the prior art. 10 to 20 wt % of the comminuted particles have a diameter of less than 1 mm, and a large fraction of this is dust. Accordingly, this fraction of comminuted particles means that 1% to 1.5% of dust is returned to the granulator per metric ton of end product, and 3-5% of the total dust per metric ton of end product from an industrial unit is attributable to the granulator.
In order to avoid or reduce the disadvantages identified above, various alternatives to formaldehyde and its water-soluble adducts and/or condensation products have been investigated, but in each case are also hampered by restrictions and/or disadvantages.
Reference may be made, for example, to the use of alkali metal lignosulfonates, as described in U.S. Pat. No. 5,766,302, or to the use of glyoxal or carbohydrates. In the resulting urea product, and depending on the production method, however these alternatives lead to a yellowish or brownish discoloration, which in many cases is undesirable. On the other hand, the use of surface-active substances such as, for example, mixtures of polyvinyl acetate and polyvinyl alcohol as granulating additives likewise leads to problems, since these additives have a tendency to foam, as for example when the additive is mixed with the melt or in the scrubbers where the treated urea dust is dissolved and impacts the efficiency of the scrubbers. The tendency of these substances to form foam also has consequences for the end product, moreover, this product having a relatively low density and being rejected by the market. Overall, therefore, any tendency toward foaming on industrial application of the urea granules is unacceptable.
Thus a need exists for a method for producing particulate, urea-containing compositions, wherein the disadvantages of the prior art are eliminated or at least diminished.